1. Field of the Invention
The present invention relates to a radio wave transmission cover, which is provided in front of a radar device for vehicles, and a method of manufacturing the radio wave transmission cover.
2. Description of the Related Art
Auto cruise systems are techniques, which measure the distance or a relative speed between a subject vehicle and a preceding vehicle, which is in front of the subject vehicle, using a sensor provided in the front end of the subject vehicle, and control a throttle or brake depending on the resultant data to accelerate or decelerate the subject vehicle, thus controlling a vehicle-to-vehicle distance. Recently, such auto cruise systems have attracted considerable attention as a core technique for constructing an intelligent transportation system (ITS), which aims to reduce the incidence of traffic jams and accidents.
Typically, laser radar or millimeter wave radar is used as the sensor for auto cruise systems. For example, millimeter wave radar transmits a millimeter wave, which has a frequency ranging from 30 GHz to 300 GHz and a wavelength ranging from 1 mm to 10 mm, to an object, and receives the millimeter wave, which has been reflected by the object. From this principle, the millimeter wave radar measures the distance or the relative speed between a subject vehicle and a preceding vehicle using the difference between the transmitted wave and the received wave.
Generally, radar devices for vehicles are disposed behind front grills of the vehicles. Such a front grill is uneven in thickness and is made of metal, or has metal-plated layers on the surfaces thereof, thus interfering the transmission of radio waves. To overcome this problem, a technique in which a window part is provided in the front grill at a position corresponding to the front end of the radar device and a radio wave transmission cover made of resin is inserted into the window part has been proposed.
The radio wave transmission cover typically has a design layer for expressing various designs. The design layer is a relatively thin layer, which is formed through vapor deposition of metal or transferring of a film. Hence, it is required to cover each of the front surface and the rear surface of the thin design layer with a reinforcing resin layer.
To manufacture the radio wave transmission cover, which is configured by covering the front and rear surfaces of the design layer with respective reinforcing resin layers, a first reinforcing resin layer is first formed, thereafter, the design layer is formed on the upper surface of the first reinforcing resin layer through vapor deposition or transfer. Subsequently, a second reinforcing resin layer is formed on the upper surface of the design layer, thus completing the manufacture of the radio wave transmission cover.
Here, to ensure a superior design of the radio wave transmission cover, the material (hereinafter, referred to as a first resin material) of the first resin layer, which is formed first, must have a melting point higher than that of the material (hereinafter, referred to as a second resin material) of the second resin layer, which is formed later. That is, in the case where the melting point of the first resin material is higher than that of the second resin material, the first resin layer, which is formed first, is prevented from being melted by the heat of the molten second resin layer. Thus, the design layer, which is formed on the upper surface of the first resin layer, is prevented from being deformed, and a radio wave transmission cover having a superior design can be produced. If the melting point of the second resin material is equal to or higher than the melting point of the first resin material, the first resin layer may be melted by the heat of the molten second resin material. Furthermore, when the first resin layer is melted, the design layer formed on the first resin layer is deformed. Therefore, in this case, it is impossible to ensure a superior design of the radio wave transmission cover.
Meanwhile, in the case where the melting point of the first resin material is higher than that of the second resin material, the compatibility between the two resin layers is reduced, with the result that it is difficult to firmly integrate the two resin layers with each other. In an effort to overcome this problem, techniques for mechanically engaging the two resin layers to each other have been proposed. (For an example, refer to Japanese Laid Open Patent Publication NO. 2000-159039).
In the radio wave transmission cover proposed in Japanese Laid Open Patent Publication No. 2000-159039, front and rear surfaces of a design layer, which is formed through the vapor deposition of indium, are covered with respective reinforcing resin layers. In detail, the front surface of the design layer is covered with a resin layer (referred to as a cover layer), which is made of transparent resin. The rear surface of the design layer is covered with another resin layer (referred to as a substrate layer), which is made of opaque resin. Furthermore, a further resin layer (referred to as a mask layer), which is made of opaque resin, is interposed between the cover layer and the design layer. The mask layer covers part of the rear surface of the cover layer. An engaging part having an undercut shape is formed on the circumferential outer edge of the cover layer.
In the radio wave transmission cover proposed in Japanese Laid Open Patent Publication No. 2000-159039, the engaging part, which is provided on the circumferential outer edge of the cover layer, is engaged to the rear surface of the circumferential outer edge of the substrate layer. Thereby, the cover layer is mechanically fastened to the substrate layer. Therefore, even though the cover layer and the substrate layer are made of materials having different melting points, the cover layer and the substrate layer can be firmly integrated with each other. Furthermore, in the radio wave transmission cover of Japanese Laid Open Patent Publication NO. 2000-159039, the design layer is visible to the front side (a cover layerside) of the radio wave transmission cover through a part (hereinafter, referred to as a window part) of the cover layer that is not covered with the mask layer. Therefore, the design of the design layer of the radio wave transmission cover is expressed in a shape corresponding to the shape of the window part.
However, in the case where the resin material of the cover layer differs from that of the substrate layer, the relative positions of the cover layer and the substrate layer may be displaced from the correct positions by the difference in contraction ratios between the cover layer and the substrate layer. In this case, although the cover layer is mechanically fastened to the substrate layer using the technique proposed in Japanese Laid Open Patent Publication NO. 2000-159039, in portions of the radio wave transmission cover other than the engaging part, the cover layer may be separated from the substrate layer. If the cover layer is partially separated from the substrate layer, gaps (air layers) are formed between the cover layer and the substrate layer, thus deteriorating the radio wave transmissibility of the radio wave transmission cover.
Furthermore, a variety of designs has been required lately in the radio wave transmission cover, but there is a limit to the design of the radio wave transmission cover of Japanese Laid Open Patent Publication No. 2000-159039. That is, because the mask layer of the radio wave transmission cover of Japanese Laid Open Patent Publication No. 2000-159039 covers only portions of the rear surface of the cover layer, the mask layer must be formed through molding or printing. However, the radio wave transmission cover of Japanese Laid Open Patent Publication No. 2000-159039 includes the engaging part having the undercut shape, and thus the mask layer cannot be formed on the engaging part or the surrounding area. Therefore, the conventional technique, proposed in Japanese Laid Open Patent Publication No. 2000-1590.39, is problematic in that a superior design of the radio wave transmission cover and a high degree of freedom in designing it cannot be ensured.